Method of forming a borehole using a compressible and noncompressible fluid in a dual pipe string

ABSTRACT

Drilling apparatus having a dual pipe string connected to a drill bit by a bit sub, with the bit sub comprising one or more subs, with the outer surface of each sub including turbulence producing means formed thereon. Compressible fluid is flow connected to the pipe annulus while drilling fluid is flow connected to the hole annulus, and the length of the bit sub is adjusted to control the pressure drop of the noncompressible fluid thereacross to thereby control the ratio of compressible and noncompressible fluid which flows up through the central passageway, carrying formation samples formed by the bit admixed therewith.

United States Patent Primary Examiner-Stephen .l. Novosad Attorney-Marcus L. Bates ABSTRACT: Drilling apparatus having a dual pipe string connected to a drill bit by a bit sub, with the bit sub comprising one or more subs, with the outer surface of each sub including turbulence producing means formed thereon. Compressible fluid is flow connected to the pipe annulus while drilling fluid is flow connected to the hole annulus, and the length of the bit sub is adjusted to control the pressure drop of the noncompressible fluid thereacross to thereby control the ratio of compressible and noncompressible fluid which flows up through the central passageway, carrying formation samples formed by the bit admixed therewith.

Patented Aug. 3, 1971 2 Sheets-Sheet.- 1

FIG. 2

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INVENTOR WAYLAND D. ELENBURG MARCUS L. BATES Patented Aug. 3, 1971 2 Sheets-Sheet 2 ENVENTGR WAYLAND D. ELENEURG BY MARCUS L. BATES METHOD OF FORMING A BOREI'IOLE USING A COMPRESSIBLE AND NONCOMPRIZSSIBLE FLUID IN A DUAL PIPE STRING BACKGROUND OF THE INVENTION Portable drilling rigs are available for cutting continuous cores, wherein the core sample is in the form of small chips such as may be produced by a cone type bit, or a cylindrical length of core as maybe obtained from a diamond bit. Generally, the cores, or subsurface samples, are returned to the surface of the earth by using dual concentric drill pipe, with drilling fluid being circulated down the pipe annulus while the core samples are returned to the surface of the earth by means of the central passageway. During this operation it is a usual practice to maintain the fluid contained in the hole annulus in a static condition. Drilling processes of this nature are more specifically discussed in my U.S. Pat. Nos. 3,439,757 and 3,473,671.

The art of raising liquids by means of compressed air is known to those skilled in the art as evidenced by the air lift device on page 2261, Chemical Engineers Handbook, Second Edition, John H. Perry; Editor, McGraw Hill Book company, lnc., New York.

in the arid regions of the United States, as for example in the southwestern portion of the Great State of Texas, water is often a difficult substance to obtain, an often a borehole will penetrate the earth with no evidence of moisture being observed for hundred and sometimes thousands of feet. Under conditions of this nature, the driller will often resort to the art of air drilling and hopefully never runs into moisture, for should he penetrate an aquifer, he must then resort to the art of drilling with mud, or a liquid, on order to economically drill a borehole. Upon resorting to liquid drilling fluid, the cost of drilling often becomes enormous for the reason that the water must be trucked to the drilling site. Moreover, the water is often lost through cracks and fissures in the earth as fast as it can be pumped into the ground. Accordingly, it is desirable to be able to conserve water when using a liquid drilling fluid in conjunction with a dual string pipa. it is furthermore desirable to be able to penetrate the earth using air drilling methods and upon discovering an aquifer, to be able to make economical use of the aquifer rather than attempt to continue with the dry drilling method. Moreover, it is highly desirable to be able to combine the liquid and the air drilling methods together into a single process which compliments each other in a manner to enhance the earth boring operation and to bring about a new, more efficient, more economical drilling method so as to cause drilling operators to successfully penetrate the earth where others have failed to economically attain this goal, especially when the cost of water becomes a major expense.

SUMMARY OF THE INVENTION This invention relates to a method of drilling bore holes using a compressible fluid such as air, a noncomprcssiblc fluid such as water, and a dual pipc string connected to a borehole forming bit by a special bit suba Air flows down the pipe cnnu= lus to the cone or cutting teeth of the bit, and then returns to the surface of the earth carrying liquid and core samples therewith in a manner analogous to an air lift=typc pump. The water flows into the hole annulus and about the drill bit sub to the drill bit where it cools the bit, aids in penetrating the earth, and provides a means by which the cuttings can be returned to the surface ofthc earth. The rate of flow about the drill bit sub is controlled by forming a series of turbulence producing rc= glons which are spaced apart and circumfcrcntlaily extend about and form a port of the outer peripheral wall surface of the drill bit sub. The length of the sub together with the con= figuration of the grooves control the rate at which liquid flows thcreacross, and accordingly affords a means by which the ratio ofthc liquid to gas can be positively controlled.

it is therefore an object of this invention to provide a new method of drilling which utilizes a mixture of s compressible and noncompressible fluid in order to transport cuttings from a drill bit to the surface of the earth.

Another object of the present invention is the provision of an earth boring process wherein means are provided by which a mixture of liquid and gas drilling fluid contained within the central passageway of a dual pipe string can be controlled downhole in a borehole.

A further object of the present invention is the provision of a method of drilling using a mixture of compressible and nonpressible drilling fluids.

A still further object of the present invention is to provide a new drilling process which requires a minimum of liquid drilling fluid.

Another object of the present invention is to provide a means by which boreholes can be formed through strata having a tendency toward caving and which are therefore normally not self-supporting.

The above objects of the present invention are attained by the provision of a dual drill pipe string attached to a formation cutting bit by a bit sub with the bit sub having turbulence producing means associated therewith, and wherein liquid drilling fluid fills the hole annulus, a compressible drilling fluid fills the pipe annulus, and the cuttings along with the compressible and noncomprcssible fluid are permitted to flew upwardly through the central passageway to the surface of the earth with the compressible to noncompressible fluid ratio being controlled by the magnitude of the turbulence producing means located on the bit sub.

These and other objects of the present invention will become apparent to those skilled in the art upon studying the following detailed description and by referring to the accompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS P10. 1 is a diagrsmrnstical, partly schcrnsticsi, cross=scc= tlonnl representation of a section of the earth within which sp= psratus fabricated in accordance with the present invention is disclosed;

FlG. 2 is a schcmuticcl representation of a theoretical con= sidcrstion of the present invention;

FIG. 3 is an enlarged cross=scctional representation of one form ofs part of the present invention, and which forms a por= tion of the apparatus soon in FIG. 1;

FIG. 4 is a cross=scctionsl view taken along line 4% of FIG. 3;

FIG. 5 is an enlarged partly cross=scctionsl. partly disgram= matical view of port of the apparatus disclosed in FIG. 1;

FIG. 6 is an enlarged cross=scctionol view of apparatus used in conjunction with FIGS. 1 and 5;

FIG. *7 is an enlarged pert cross=scctionol view of apparatus used on conjunction with F: 1 and 5;

FIG. 8 is an enlarged partly cross=scctlonai part schcmaticci rcrrcscntction of part of the apparatus soon in FIGS. I and 3: an

FIGS. 9 end 19 are cross=scctional views taken along lines 9 and 10, respectively, of FIG. 5.

DETAILED DESCRIPTIQN GP THE VARIQUS EMBODIMENTS Looking now to the details ofthc drawings, and in particular to FIG. 1 which sets forth the essence of the drilling method or the present invention, the earth 10 is soon to have a surface 11 from which there downwardly depends a borehole 12, the upper marginal end portion thereof having been provided with the usual surface casing 14. Positioned within the borehole there is a dual pipe string id comprised of an outer pipe l6 and an inner pipe 17 which is concentrically disposed therein. all of which may be fabricated in accordance with Hendersons U.S. Pat. No. 3,208,539; although, other concentrically arranged pipe could be substituted therefor while still remaining within the comprehension of the present invention.

The dual pipe string cooperates with the borehole to form borehole annulus 18, pipe annulus l9, and a central longitudinally extending passageway 20. At the lower extremity of the drill pipe there is seen an adapter sub 22, having upper and lower end portions 21 and 23, which forms a drill bit sub, all of which is made in accordance with the present invention, and to the lower extremity of which there is connected a bit 24 which penetrates the earth at 25 to form the borehole. While a cone-type bit is disclosed, bits of other configurations are contemplated by this invention.

At the upper extremity of the pipe string there is connected a swivel 26 of conventional design, which is provided with an air inlet 27 connected to the pipe annulus, with a compressor 28 supplying the source of air thereto. Outlet conduit 29 is connected to the inner tubing, or central passageway of the drill string, for delivering cuttings, air, and drilling fluid to a mud pit or water reservoir 31. The mud pit contains the usual addatives, gels, and detergents peculiar to the particular formation being penetrated and to the specific gravity required of the fluid.

The mud pit is connected to the hole annulus by means of flow passageway 32 which extends through or into the surface casing so as to flood the annulus with an abundance of noncompressible liquid. Located downhole there is seen a cavity 33 within the formation through which fluid could flow so as to escape the annulus, under some conditions.

Both the pipe and the inner tubing are made up of a multiplicity of joints, with a seal means being provided between each joint in accordance with the before mentioned Henderson patent, for example. Numeral 35 schematically indicates sir flow and it will be noted that the small indicia or pictorial representation relates to highly compressed air. At 36 it will be noted that the air flow is under relatively less pressure and accordingly the indicis is larger. Numeral 37 indicates cuttings which are suspended in the compressible and noncompressible fluid mixture as it is forced upwardly through the central passageway of the drill string.

Looking now to the details of FIG. 2, in conjunction with the remaining flgures there is seen u schematics! representation of the flow of noncompresslble fluid as it occurs across the bit sub during its travel from the hole annulus to the bit. The bit sub 12! includes a series oi grooves 39 which are spaced apart from one another and circumfercntially disposed theresbout where it forms part of the outer peripheral surface area of the bit sub. The purpose of the grooves is to form a tur= bulence producing region along the outer surface of the bit sub. The letters V-l indicate the fluid velocity between an upper pair of adjacent grooves, while V=2 indicates the velocity within the enlargement provided by the nest lowcr groove, while V-3 indicates the velocity oitho fluid as it flows towards the next and lower adjacent groove. it should be noted that the annulus bctwocn bit sub 12! and bore hole in in the order of a few thousandths of an inch. The size of the groove is in the order of ono=cighth to ono=fourth inch diamo= tar, depending upon the outside diameter ofthc sub.

Looking now to the details of FIG. 3 there is disclosed a bit sub 121 having a box or throsdcd connection 116 adapted to be affixed to the lower depending end of the drill pipe. inner tubing connector has a reduced diameter 134 for receiving the lowermost end portion of the inner tubing in scaled rola= tlonship therewith. Grooves 39 are circumtcrontially disposed about and form a portion of tho outermost surface area of tho bit sub. A bit 24 is slidsbly rscoivcd within the downwardly do: pending skirt 4! with the skirt having a lowermost and portion thereof and to which there may be supplied high abrasive material 42 if desired.

The inner tubing adaptor forms passageway so which com= municstcs with passageway 44 formed within portion 45 otthc bit. The bit throsdedly engages tho bit sub as indicated at so. The circumfcrsntislly disposed annular passageway it? coin= municatcs the bit cone 24' with the pipe annulus by means of spaced apart flow passageways 48 and 49. Where doomed desirable, the area between the shanks to which the cones are attached may be provided with a bafile 50, the details of which are more fully discussed in my above-mentioned patents. Flow passageway 51 enables air to flow from annular chamber 47, across the cutting surface of the bit, and back up through the central passageway 44 and on the inner tubing.

The bit sub 121 can be varied in construction in order to accommodate any reasonable configuration and size of bit and plpestring combination desired.

Looking now to the details of FIG. 5 in conjunction with the remaining figures there is disclosed therein a drill bit sub 53 which may be used in conjunction with a drill bit 24. The upper extremity of the sub is provided with threads 216 and tubing adapter 234 which receives the concentrically arranged drill pipe therein in the same manner of FIG. 3. Drill bit 24 is attached to the lower extremity of the sub by engaging threads 46 with the complimentary threads provided at 146. With the sub interposed between the drill bit and drill pipe, passageway 149 interconnects the pipe annulus with the circumferentially disposed passageways 147 which in turn provides flow of compressible fluid at 148 to the bit. Skirt 141 receives the bit therein in a manner similar to the disclosure of FIG. 3, while edge portion 160 abuts a shoulder provided on the bit. The lower extremity of the skirt at 161 enclosed the bit shank in a manner similar to the illustration of FIG. 3. Pussagcway provides fluid flow to the central tubing from passageway 44 of the bit. Circumferentially spaced apart grooves 139 are disposed about and form a portion of the outer peripheral wall surface of the sub.

As seen in the illustrative example of FIG. 6 the sub 53 can be lengthened by interposing member 54 between the drill stem and the upper extremity of sub 53 by merely engaging the threaded connection at pin 246 with the complementary threaded connection or box 216 while the connection at 216' and 234' receives the lower terminal end of the drill stem portions in the usual manner. The pipe annulus is now flow connected to passageway 249 which in turn is flow connected to passageway 149, and the drill string central passageway is aligned with passageways 220 and 120.

Located at the upper extremity of subs 53 and 54 there is radially disposed at a set of up=roomors 164 or 264, rcspcctive= ly. The outer surface of the rcsmers is provided with high abrasive material such as tungsten carbide, for example.

As further indicated by FIG. 7 a bit sub 55 can be inter= posed between sub 53 and the bit by throadodly engaging pin 35? with threads 146 with shoulder making up with the ll= lustrstcd shoulder 358 located adjacent to the pin of the bit sub 33. Passsgewsy 349 communicates with passageway 148 while the lower shoulder 161 abuts shoulder 36% cftho sub as= scmbly 55. The pin of the bit is received by threads 346 with shoulder 360 abutting the illustrated shoulder on the bit. The lower depending portion 361 of skirt 341 now clrcurn= forcntiolly enclosed the shanks of the bit. as in the preceding example of FlG. 3.

Those skilled in the art will realize that still additional sub assemblies. such as soon at 54 in FlG. 6 and 55 in FIG. 7 can be interposed between the bit ad the drill string.

CPERATlON The drilling opcration as disclosed in FIG. 1 is preferably commenced using air as the drilling fluid until the borehole is of sufficient depth to cement in the surface casing. or until a moisture bearing strata is cncountcrcd. At this time it may be necessary to open passageway 33 in order to admit sufficient liquid to provide the required hydrostatic hood in the borehole annulus, ad a bit sub 22, comprised of subs 121, 53. or 54 is placed bctwocn the drill pipe and the drill bit. The drilling operation is resumed with air from 28 flowing down the pipe annulus and to the bit whereupon the air ndmixes with the liquid contained within the control passageway, causing considerable difference between the hydrostatic head of the fluid columns of the borehole and pipe annulus. The liquid and air transports the chips formed by the bit in an upward direction through the central passageway and to the screen 30 where the chips or formation cuttings are retained as samples while the fluid is returned to the mud pit 31. The rate of liquid return is governed by the flow rate at 27 together with the pressure drop across the bit sub. As the borehole deepens, the increased hydrostatic pressure differential causes excessive flow across the bit sub, and accordingly it becomes desirable to increase the pressure drop thereacross in order to reduce the consumption of water, that is, to change the gas/liquid ratio. This expedient is accomplished by elongating the sub by interposing additional subs 54 between sub 53 and the tubing string, or alternatively, by interposing an additional sub 55 between sub 53 and the bit.

The pressure drop across the sub is proportional to the length of the sub as well as to the number and configuration of the grooves located therein. The grooves enchance turbulent flow in that they cause the liquid flowing thereacross to pursue an erratic flow path. For example, referring to FIG. 2, it will be seen that liquid at V-l, upon entering a groove, will suffer from an expansion loss analogous to a condition where fluid passes an obstruction. In other words, as the fluid flows from V 1 to V-2 it forms a vena contracta"; that is, a contraction of the fluid flow stream is formed which causes a major friction loss to arise from the excessive turbulence induced as the fluid flows into the more slowly moving fluid at V-2. Accordingly, for each pound mass of fluid flow a mechanical energy loss is efiected which is proportional to the number of grooves the fluid must traverse. Therefore, the velocity at V-l is much greater than the velocity at V-2 and the velocity at V- 3 must be greater than the velocity at V-2, with the resulting mechanical energy loss being attributable to the accumulative increased turbulent flow. Stated differently, a friction loss is brought about due to the fluid being forced from a straight line to an erratic flow path as the fluid is forced to traverse each of the grooves.

By retarding fluid flow across the sub assembly, the air to water, or compressible to incompressible fluid ratio can be maintained at its optimum value so far as regards the usage of water and the fluid velocity in the central tubing.

The air contained within the pipe annulus is compressed and accordingly very little friction loss is engaged as the air travels from the swivel to the hit. As the air enters the inner barrel of the bit it expands, admixes with the liquid, and reduces the hydrostatic head of the central column of fluid, thereby causing the fluid to move up through the central passageway since the last named column of fluid has an apparent density much less than the hydrostatic head contained within the hole annulus. The density can be regulated in accordance with the liquid/air ratio as well as the composition and viscosity of the mud. For example, in a static system using a 8.3 pound fluid, a 1,000 foot hydrostatic head normally would exert about 400 pounds pressure as measured at the bit. Aeration of this column of fluid reduces its apparent density and at the surface it may have a value approaching 0.85 pounds, indicating that a to I change in its apparent density has been achieved.

When using a 4% inch outside diameter drill string, drill bits and subs ranging up to 10 inches outside diameter can he used with the present invention. The grooves for a 7% inch sub are preferably three-eighths inch in diameter and spaced approximately 1 inch apart. As the drill bit sub is further increased in diameter, the diameter of the grooves are likewis increased, Grooves one-eighth inch in diameter have also prqven to be efiective in conjunction with a 7% inch in diameter and sub.

While forming a bore hole, should it become apparent that excessive water is being produced, it is preferred to continue the drilling operation until it is necessary to make another trip out of the hole for replacement of the drill bit, whereupon additional sub assemblies can then be added between the bit and the drill pipe.

When returning into the hole, as for example, a IOOO-foot hole, it would ordinarily appear necessary to provide 400 I pounds pressure in order to commence initial aeration of the central passageway. This magnitude of pressure, that is, a pressure equivalent to the static hydrostatic head, is not required for the reason that many of the joints located along the inner tubing will be leaker," that is, the o-ring seal (see the Henderson patent) will inherently leak a sufiicient amount to cause step-by-step aeration of the liquid column in a downward direction as the pressure is increased at 28, and accordingly, it has been found that by gradually increasing the pressure at 17, only about pounds per square inch pressure is required in order to commence the drilling operation. The inherent leakers located along the inner tubing connections eliminate the initial high pressure requirements ordinarily associated with most air lift devices, such as used in water wells, for example. Stated difi'erently, a sufficient number of O-ring seals located between each joint of inner tubing will act as spaced apart preloaded check valves which progressively aerate the central liquid column in a downward direction whereby the central column of liquid progressively is aerated to thereby drastically reduce the initial pump pressure to a valve which is less than the static hydrostatic head.

This phenomenon is naturally more pronounced as the inner tubing o-ring seals become worn or damaged through continual usage. Accordingly, the inherent ability of a portion of the O-rings to leak provides a means for aerating the central liquid column in a manner analogous to the spaced-apart valves associated with a gas lift tubing string as is known in the oil production art. Therefore it is considered within the comprehension of the this invention to provide preloaded check valves in spaced-apart relationship along the inner tubing which open at a predetermined pressure level in order to attain the above defined bypassing of compressible fluid from the pipe annulus to the central tubing, although such an expedient has been found unnecessary in the drilling processes encountered thus far.

The present process enables faster penetration of a strata for the reason that the pressure differential across the bit sub produces a lower pressure at the borehole bottom, thereby causing or enabling the formation to relieve stresses therein in an upward direction, all of which reduces the work required of the bit, Where the borehole wall tends to cave" or fall inwardly, a heavy mud can be employed to overcome this undesirable condition. The mud will coat and reinforce the wall structure as well as providing outwardly directed forces due to its hydrostatic head. Therefore the noncompressible fluid used in conjunction with the present invention can be water or any of the drilling muds known to those skilled in the art. The compressible fluid can be air, natural gas, inert gas, or any other desired compressible fluid available to the driller.

Moreover, others skilled in the art having read this disclosure, will recognize the desirability of utilizing other forms for controlling the pressure drop across the bit sub, as for example: means for effectively controlling the diameter of the sub as well as other configurations of the groove geometry, and such changes are considered to be design considerations which lie within the comprehension of this disclosure.

lsla mz 1. Method of drilling boreholes with a bit connected to a dual pipe string by a bit sub, comprising the steps of:

flooding the borehole annulus with a noncompressible r lli g fluid;

forcing a compressible fluid down the drill pipe annulus to the bit;

flowing the compressible fluid, noncompressible drilling fluid, and cuttings from the bit uphole through the central p ageway of the dual pipe string;

cont lling the ratio of compressible fluid to noncompressible drilling fluid by controlling the pressure drop of the noncompressible drilling fluid as it flows from the hole annulus across the cutting surface of the bit.

2. The method of claim 1, wherein control of the pressure drop is effected by forming a series of turbulence producing regions on a drill bit sub by forming spaced-apart grooves which are circumferentially disposed about the outer peripheral surface of the bit sub.

3. The method of claim 1 wherein the pressure drop is controlled by increasing the friction loss of the drilling fluid as it flows across a bit sub by cutting turbulence inducing grooves about the outer surface of the bit sub.

4. The method of claim 1 wherein the pressure drop is controlled by forming turbulence producing mean in the outer peripheral surface of a bit sub, and

. lengthening the portion of the sub containing the turbulence producing means in order to further increase the pressure drop thereacross.

5. The method of claim 4 wherein the turbulence producing region of the sub is lengthened by adding several subs together in series relationship in order to form a longer turbulence producing means.

6. The method of claim 1 wherein the fluid contained within the central passageway is admixed with compressible fluid at intermediate locations along the length of the borehole by allowing the compressible fluid contained within the drill string annulus to flow from the pipe annulus into the central passageway.

7. The method of claim 6 wherein pressure drop is controlled by providing the outer peripheral surface of a bit sub with turbulence producing means, and lengthening the portion of the sub containing the turbulence producing means in order to increase the pressure drop thereacross.

8. The method of claim 6 wherein the pressure drop is controlled by increasing the friction loss of the drilling fluid as it flows across a bit sub by cutting turbulence inducing grooves about the outer surface of the bit sub.

9. A method of drilling a borehole with drilling apparatus having a dual drill pipe string connected to a drill bit by a bit sub, and with the bit forming a borehole in the earth, and with the borehole and pipe string cooperating together to form a hole annulus, a pipe annulus, and a central passageway comprising the steps of:

1. flow connecting a supply of liquid drilling fluid to the hole annulus;

2. flow connecting a supply of compressible fluid to the pipe annulus;

3. rotating the bit by the pipe string to cause the bit to form cuttings while penetrating the earth and forming the borehole;

4. maintaining the hydrostatic head of the central tubing at a value which is less than the hydrostatic head of the hole annulus by flowing the compressible fluid of step (2) through and about the bit;

5. circulating the cuttings, the compressible fluid, and the drilling fluid up through the central passageway and to the surface of the earth in order to obtain samples of the strata being penetrated by the bit.

10. The method of claim 9 and further including:

flowing a portion of the compressible fluid located in the pipe annulus from the pipe annulus into the central passageway, with the flow occurring at a plurality of locations between the bit sub and the surface of the earth to thereby reduce the initial pressure requirements when commencing the drilling operation.

11. The method of claim 10 wherein the pressure drop across the bit sub is controlled by increasing the friction loss of the drilling fluid as it flows across the bit sub by cutting turbulence inducing grooves about the outer surface of the bit sub.

12. The method of claim 9 wherein the pressure drop is controlled by providing the outer peripheral surface of the bit sub with turbulence producing means, and lengthening the portion of the bit sub containing the turbulence producing means in order to increase the pressure drop thereacross.

13. The method of claim 12 wherein the sub is lengthened by adding several subs together in series relationship so as to form an increased turbulence producing means. 

2. flow connecting a supply of compressible fluid to the pipe annulus;
 2. The method of claim 1, wherein control of the pressure drop is effected by forming a series of turbulence producing regions on a drill bit sub by forming spaced-apart grooves which are circumferentially disposed about the outer peripheral surface of the bit sub.
 3. The method of claim 1 wherein the pressure drop is controlled by increasing the friction loss of the drilling fluid as it flows across a bit sub by cutting turbulence inducing grooves about the outer surface of the bit sub.
 3. rotating the bit by the pipe string to cause the bit to form cuttings while penetrating the earth and forming the borehole;
 4. The method of claim 1 wherein the pressure drop is controlled by forming turbulence producing mean in the outer peripheral surface of a bit sub, and lengthening the portion of the sub containing the turbulence producing means in order to further increase the pressure drop thereacross.
 4. maintaining the hydrostatic head of the central tubing at a value which is less than the hydrostatic head of the hole annulus by flowing the compressible fluid of step (2) through and about the bit;
 5. circulating the cuttings, the compressible fluid, and the drilling fluid up through the central passageway and to the surface of the earth in order to obtain samples of the strata being penetrated by the bit.
 5. The method of claim 4 wherein the turbulence producing region of the sub is lengthened by adding several subs together in series relationship in order to form a longer turbulence producing means.
 6. The method of claim 1 wherein the fluid contained within the central passageway is admixed with compressible fluid at intermediate locations along the length of the borehole by allowing the compressible fluid contained within the drill string annulus to flow from the pipe annulus into the central passageway.
 7. The method of claim 6 wherein pressure drop is controlled by providing the outer peripheral surface of a bit sub with turbulence producing means, and lengthening the portion of the sub containing the turbulence producing means in order to increase the pressure drop thereacross.
 8. The method of claim 6 wherein the pressure drop is controlled by increasing the friction loss of the drilling fluid as it flows across a bit sub by cutting turbulence inducing grooves about the outer surface of the bit sub.
 9. A method of drilling a borehole with drilling apparatus having a dual drill pipe string connected to a drill bit by a bit sub, and with the bit forming a borehole in the earth, and with the borehole and pipe string cooperating together to form a hole annulus, a pipe annulus, and a central passageway comprising thE steps of:
 10. The method of claim 9 and further including: flowing a portion of the compressible fluid located in the pipe annulus from the pipe annulus into the central passageway, with the flow occurring at a plurality of locations between the bit sub and the surface of the earth to thereby reduce the initial pressure requirements when commencing the drilling operation.
 11. The method of claim 10 wherein the pressure drop across the bit sub is controlled by increasing the friction loss of the drilling fluid as it flows across the bit sub by cutting turbulence inducing grooves about the outer surface of the bit sub.
 12. The method of claim 9 wherein the pressure drop is controlled by providing the outer peripheral surface of the bit sub with turbulence producing means, and lengthening the portion of the bit sub containing the turbulence producing means in order to increase the pressure drop thereacross.
 13. The method of claim 12 wherein the sub is lengthened by adding several subs together in series relationship so as to form an increased turbulence producing means. 